If natural remedies for your algae problem didn’t work to your satisfaction, there are chemical remedies that can be exercised with caution.

Remove as much algae as possible before starting chemical treatment. Maximum effectiveness of ridding algae occurs at 50 degrees Fahrenheit.

Just as sunlight will feed plants as well as algae, algicides will slow the growth of algae as well as other plants and high doses may kill them. A way of avoiding too high dosages is to perform a fifty percent water change and then treat the water with a half dose.

The algicide Potassium Permanganate is only to be used on cool, cloudy days when the water isn’t too warm. If used on sunny days, the treatment will turn the water a murky yellow. A maximum of three applications on three successive days is usually recommended. At the end of the treatment remember to remove the dead algae.

Formaldehyde (37 percent solution) may be used in the rate of one drop per gallon of pond water to kill floating algae. A possible killer of lily cultivators, it is wise to remove plants before treatment. Fish will not need to be moved as the 37 percent solution is safe for killing fish parasites but will not kill the fish. The person handling the solution should avoid skin contact and breathing in the chemicals.

For larger bodies of water, Copper sulfate can be used. Dosages vary and will usually be used on alternate days over a two week period if there are fish in the pond. Use Copper sulfate with great caution because a too high dosage can kill your fish by way of asphyxiation.

Simazine is a more gentle option for smaller ponds. If used properly, it is safe for goldfish and koi. However, it will affect or kill more sensitive fish and plants. Therefore, all plants and applicable fish should be moved to a holding tank during treatment. Extra aeration should be supplied as oxygen levels will be affected for several weeks after completed.

Like Simazine, the herbicide Urea Maleate is deemed safe for koi and goldfish but will also affect or kill other sensitive fish and plants.

Before aquatic plants have reached a size that can compete with algae, a (non-toxic) blue or black dye can be added to the pond to reduce the amount of sunlight entering the pond. Submerged plants and lilies should be moved closer to the surface so as to get a better access to the sunlight that is entering. The dye will gradually be removed through routine water changes and time.

Pond Balance, a product that changes the chemistry of your pond water, makes conditions unsuitable for the growth of filamentous algae. This product is safe for all pond life but will generally need to be used on a regular basis throughout the season, usually three full doses in both autumn and spring.